Materials Letters 64 (2010) 2742–2744 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t Improved interfacial properties of carbon ﬁber/epoxy composites through grafting polyhedral oligomeric silsesquioxane on carbon ﬁber surface Feng Zhao, Yudong Huang ⁎ Department of Polymer Science and Engineering, Harbin Institute of Technology, PO Box 410#, Harbin 150001, China a r t i c l e i n f o Article history: Received 16 July 2010 Accepted 29 August 2010 Available online 15 September 2010 Keywords: Carbon ﬁber POSS Surfaces Composite materials a b s t r a c t Carbon ﬁbers were grafted with a layer of uniform octaglycidyldimethylsilyl POSS in an attempt to improve the interfacial properties between carbon ﬁbers and epoxy matrix. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and dynamic contact angle analysis were performed to characterize the carbon ﬁbers. AFM results show that the grafting of POSS signiﬁcantly increased the carbon ﬁber surface roughness. XPS indicates that oxygen-containing functional groups obviously increased after modiﬁcation. Dynamic contact angle analysis shows that the surface energy of modiﬁed carbon ﬁbers is much higher than that of the untreated ones. Results of the mechanical property tests show that interlaminar shear strength (ILSS) increased from 68.8 to 90.5 MPa and impact toughness simultaneously increased from 2.62 to 3.59 J. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Carbon ﬁbers are widely used as reinforcements of advanced composites because of their excellent properties, such as high speciﬁc strength and modulus, light weight and relative ﬂexibility . However, when applied without previous surface treatment, the physicochemical interaction between carbon ﬁbers and matrix is not strong enough due to their inert and smooth surfaces, which will directly affect the interfacial adhesion of the composites . As a result, extensive research has been devoted to the surface treatment of carbon ﬁbers in order to increase the quantity of surface functional groups and enhance the interactions between ﬁbers and matrix, such as oxidation method , electrochemical method , plasma treatment , and high energy irradiation . Polyhedral oligomeric silsesquioxanes (POSS) are emerging as a new chemical technology for the nano-reinforced organic-inorganic hybrids, which have been applied in diverse areas including aerospace, semiconductor and biological systems, and becoming the focus of many studies due to the simplicity in processing and the excellent comprehensive properties . Although grafting POSS on the carbon ﬁber surface is a potential modiﬁcation method, we could not ﬁnd any reports on this in a literature survey. Here, we grafted octaglycidyldimethylsilyl POSS on the carbon ﬁber surface through a series of chemical reactions to enhance the interfacial adhesion between carbon ﬁbers and matrix. The surface chemical composition and morphologies of carbon ﬁbers were investigated by XPS and AFM. Wettability and surface energy of the ⁎ Corresponding author. Tel./fax: + 86 451 86413711. E-mail address: email@example.com (Y. Huang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.08.074 carbon ﬁbers were obtained by dynamic contact angle analysis. The mechanical properties of composites were evaluated by ILSS and impact toughness. 2. Experimental PAN-based carbon ﬁbers were purchased from Sinosteel Jilin Carbon Co., China. Octaglycidyldimethylsilyl POSS was purchased from Hybrid Plastics. Thionyl chloride (SOCl2), ethylenediamine (EDA), dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Sigma-Aldrich. The carbon ﬁbers were oxidized in a 3:1 (v/v) mixture of concentrated H2SO4/HNO3 at 60 °C for 2 h. Then the carboxyl functionalized carbon ﬁbers were reacted with the mixture solution of 50 ml SOCl2 and 5 ml DMF at 76 °C for 24 h to yield acyl chloride functionalized carbon ﬁbers. Then the ﬁbers were reacted with 50 ml EDA at 80 °C for 24 h. After being washed with deionized water and dried, the amine functionalized carbon ﬁbers were mixed with 0.5 g POSS in 100 ml THF reacting at 50 °C for 6 h to obtain POSS grafted carbon ﬁbers. WSR618 epoxy resin and methyl tetrahyelrophthalic anhydride hardener were supplied by Sinopharm Chemical Reagent Co., used at a mixture ratio of 100:70. The unidirectional prepreg of carbon ﬁbers was put into a mold to manufacture composites. The curing process was at 90 °C for 2 h under 5 MPa, 120 °C for 2 h under 10 MPa and 150 °C for 4 h under 10 MPa. The resin content of the composites was controlled at 35 ± 1.5 mass%, and the width and thickness of specimens were 6.5 and 2 mm. The surface composition analysis was performed on a Scienta ESCA 300 XPS. AFM images were obtained using a NT-MDT Solver P47H system. Dynamic contact angle analysis was performed on a DataPhysics DCAT21 dynamic contact angle meter and tensiometer. Mechanical properties were tested on Instron 5569 universal testing F. Zhao, Y. Huang / Materials Letters 64 (2010) 2742–2744 2743 Table 1 Surface element analysis of carbon ﬁbers. Carbon ﬁber As-received POSS grafted Element content (%) C O N Cl Si 84.88 66.46 13.50 23.23 1.62 1.90 – 0.10 – 8.31 O/C Si/C 0.1590 0.3495 – 0.1250 Fig. 1. Structure of octaglycidyldimethylsilyl POSS. machine and Instron 9250HV drop weight impact test system. The structure of octaglycidyldimethylsilyl POSS is shown in Fig. 1. 3. Results and discussions The AFM images of untreated, POSS grafted carbon ﬁbers are shown in Fig. 2. Remarkable differences of the surface topography can be observed between the untreated and modiﬁed carbon ﬁbers. As shown in Fig. 2a, the surface of the untreated carbon ﬁber seems to be relatively neat and smooth, and a few narrow grooves parallel distribute along the longitudinal direction of the ﬁber. After modiﬁcation, a layer of POSS particles are grafted uniformly on the ﬁber surface and the roughness increases obviously (Fig. 2b). The increased surface roughness can signiﬁcantly increase the interfacial adhesion by enhancing mechanical interlocking between the ﬁber and the matrix. The results of the carbon ﬁber surface composition obtained by XPS were summarized in Table 1. It is found that the elements of the untreated carbon ﬁber surface only include carbon, oxygen and insigniﬁcant amount of nitrogen. After being grafted with POSS, the carbon content decreased from 84.88% to 66.46% and the oxygen content increased signiﬁcantly from 13.50% to 23.23%. In addition, signiﬁcant silicon elements of 8.31% were detected on the ﬁber surfaces, and surface atomic O/C and Si/C ratios increased sharply, which were due to the Si–O cage structure and the epoxy groups of POSS. These numerous epoxy groups can effectively increase resin compatibility and react with matrix. The changes of chemical environment and topography of carbon ﬁber surfaces affect the ﬁber surface energy as well as its components. In Table 2, the advancing contact angle (θ), the surface energy (γ), its dispersion component (γd) and polar component (γp) of the untreated and POSS grafted carbon ﬁbers are summarized. As shown in Table 2, the surface energy of untreated ﬁbers was 43 mN m− 1, with a dispersion component of 36 mN m− 1 and a polar component of 7 mN m− 1. After modiﬁcation, obvious decreasing trends of contact angles were observed from the untreated ﬁbers to the POSS modiﬁed ﬁbers for both the polar water and the non-polar diiodomethane. The contact angles decreased from 73.21 to 53.68° for water and from 46.85 to 42.52° for diiodomethane. In addition, the surface energy and its components of POSS grafted carbon ﬁbers obviously increased compared with those of the untreated ﬁbers. The increased polar component of modiﬁed ﬁbers was due to the epoxy groups of POSS on the ﬁber surface, and the increased dispersion component was due to the increased roughness caused by POSS particles and the different surface composition of carbon ﬁbers. The increased surface energy can effectively improve the wettability of the ﬁbers by the resin and increase the interfacial strength. The mechanical property testing results of the composites reinforced by different carbon ﬁbers are shown in Fig. 3. From Fig. 3a, it can be clearly seen that the grafting of POSS signiﬁcantly increased the interfacial strength of the composites. The ILSS increased from 68.8 to 90.5 MPa by 31.5%. The improvement of the interfacial strength could be attributed to the enhancement of the mechanical interlocking and chemical bonding between the ﬁbers and matrix. After modiﬁcation, the epoxy groups of POSS play an important role in improving the interfacial adhesion between the ﬁbers and matrix. In addition, the rigid POSS particles grafted on the ﬁber surfaces can greatly enhance the mechanical interlocking with the resin. The impact property testing results are shown in Fig. 3b. The initial, propagative and total absorbed energy of untreated carbon ﬁber composites were 0.45, 2.17 and 2.62 J, respectively. After modiﬁcation, the impact properties of POSS grafted carbon ﬁber composites increased. The initial, propagative and total absorbed energy increased to 0.71, 2.89 and 3.59 J, by 57.8%, 33.2% and 37.0%, respectively. When the composites are under load, POSS in the composite interface can induce more cracks which can efﬁciently absorb the fracture energy, resulting in the increase of the initial absorbed energy. After crack formation, POSS can efﬁciently change the direction of the crack propagation, which increases the propagative absorbed energy. In a word, the stress concentration around POSS, the inducement of cracks Table 2 Contact angles and surface energy of carbon ﬁbers. Fig. 2. AFM topography images of carbon ﬁbers. a) untreated, and b) POSS grafted. Carbon ﬁber θwater (°) θdiiodomethane (°) γd (mN m− 1) γp (mN m− 1) γ (mN m− 1) As-received POSS grafted 73.21 53.68 46.85 42.52 36.00 38.32 7.00 16.55 43.00 54.87 2744 F. Zhao, Y. Huang / Materials Letters 64 (2010) 2742–2744 between carbon ﬁbers and epoxy matrix. The roughness of carbon ﬁber surface signiﬁcantly increased after being grafted with octaglycidyldimethylsilyl POSS. These POSS particles on the ﬁber surface provided a means to enhance the mechanical interlocking with the resin. XPS results indicate that the oxygen-containing functional groups obviously increased after modiﬁcation, which is beneﬁcial to increase the chemical bonding between carbon ﬁbers and matrix. The surface energy of the modiﬁed ﬁbers is much higher than that of the untreated ones, which can lead to better wettability by the resin and better interfacial adhesion. The mechanical performance tests show that the introduction of octaglycidyldimethylsilyl POSS in the interface region can obviously improve the interfacial strength and simultaneously increase the impact toughness. This signiﬁcant improvement of mechanical properties is obtained without any effort to optimize the grafting reaction conditions, so it is expected that the interfacial properties can be further increased. Acknowledgement The authors gratefully acknowledge ﬁnancial supports from the Chang Jiang Scholars Program, the National Natural Science Foundation of China (No. 51073047) and the National Natural Science Foundation of China (No. 51003021). References Fig. 3. Mechanical properties of the composites reinforced by untreated and POSS grafted carbon ﬁbers. a) ILSS and b) impact toughness. and the crack propagation orientation deﬂection are helpful to improve the impact properties of the composites. 4. Conclusion Carbon ﬁbers were grafted with a layer of uniform octaglycidyldimethylsilyl POSS in an attempt to improve the interfacial properties  Paiva MC, Bernardo CA, Nardin M. Mechanical, surface and interfacial characterisation of pitch and PAN-based carbon ﬁbres. 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